Microcrystalline cellulose is a purified, partially depolymerized cellulose that is produced by treating a source of cellulose, preferably alpha cellulose in the form of pulp from fibrous plant materials, with a mineral acid, preferably hydrochloric acid. The acid selectively attacks the less ordered regions of the cellulose polymer chain thereby exposing and freeing the crystalline sites which form crystallite aggregates which constitute the microcrystalline cellulose. These are then separated from the reaction mixture, and washed to remove degraded by-products. The resulting wet mass, generally containing 40 to 60 percent moisture, is referred to in the art by several names, including hydrolyzed cellulose, hydrolyzed cellulose wetcake, level-off DP cellulose, microcrystalline cellulose wetcake or simply wetcake.
When the wetcake is dried and freed of water the resulting product, microcrystalline cellulose, is a white, odorless, tasteless, relatively free-flowing powder, insoluble in water, organic solvents, dilute alkalis and acids. For a fuller description of microcrystalline cellulose and its manufacture see U.S. Pat. No. 2,978,446. The patent describes its use as a pharmaceutical excipient, particularly as a binder, disintegrant, flow aid, and/or filler for preparation of compressed pharmaceutical tablets. Microcrystalline cellulose is manufactured by FMC Corporation and sold under the designation Avicel.RTM. PH cellulose in several grades having average particle sizes ranging from about 20 .mu.m to about 100 .mu.m.
Microcrystalline cellulose and/or hydrolyzed cellulose wetcake has been modified for other uses, notably for use as a gelling agent for food products, a thickener for food products, a fat substitute and/or non-caloric filler for various food products, as a suspension stabilizer and/or texturizer for food products, and as an emulsion stabilizer and suspending agent in pharmaceutical and cosmetic lotions and creams. Modification for such uses is carried out by subjecting microcrystalline cellulose or wetcake to intense attrition forces as a result of which the crystallites are substantially subdivided to produce finely divided particles. However, as particle size is diminished, the individual particles tend to agglomerate or hornify upon drying, probably due to the hydrogen or other bonding forces between the smaller sized particles. To prevent agglomeration or hornification, a protective colloid, such as sodium carboxymethylcellulose (CMC), which wholly or partially neutralizes the bonding forces which cause agglomeration or hornification, may be added during attrition or following attrition but before drying. This additive also facilitates re-dispersion of the material following drying. The resulting material is frequently referred to as attrited microcrystalline cellulose or colloidal microcrystalline cellulose. For a fuller description of colloidal microcrystalline cellulose, its manufacture and uses, see U.S. Pat. No. 3,359,365, in which it is stated that at least 1% and preferably at least 30% of the microcrystalline cellulose has been reduced to a particle size not exceeding about 1.0 micron.
Colloidal microcrystalline cellulose is a white odorless, hygroscopic powder. On being dispersed in water, it forms white, opaque thixotropic gels. It is manufactured and sold by FMC Corporation (FMC) in various grades under the designations, among others, Avicel.RTM. RC and Avicel.RTM. CL, which comprise co-processed microcrystalline cellulose and carboxymethylcellulose sodium. In FMC Product Bulletin RC-16, the grades designated as RC-501, RC-581, RC-591, and CL-611 are described as producing dispersions in which approximately 60% of the particles in the dispersion are less than 0.2 micron when properly dispersed.
With increased interest in the use of finely divided cellulosic materials in food and pharmaceutical suspensions, researchers in the field have focused a significant amount of attention on improving the smoothness and mouth feel of suspensions made from such cellulosic materials, and have determined, for example, that cellulosic particles which have a particle size above about 3 .mu.m are perceived by the tongue as particulate or grainy matter. Komuro, European Patent Publication No. 0 415 193 A2, teaches that to avoid that sensation it is necessary to provide a material in which 50% cumulative volume of the cellulosic particles have a particle size in the range of 0.3 .mu.m to 6.0 .mu.m when at least 25% of the cumulative volume in the suspension has a particle size of no greater than 3 .mu.m. The patent also teaches a grinding method and apparatus which employs a high speed rotary grinder for wet grinding the cellulose material using ceramic or metallic beads as grinding medium in order to achieve these objectives. It and a related patent to Komuro et al., U.S. Pat. No. 5,123,962, but in particular the latter, have an extensive discussion on the unsuccessful efforts made in the art to further reduce the particle size of microcrystalline cellulose.
Similarly, U.S. Pat. No. 5,415,804, teaches that a smooth mouth feel depends on the colloid fraction as well as particle size distribution and average particle size, the colloid fraction being that fraction of the dispersed particles which cannot be precipitated upon centrifugation of the dispersion. Depending on the particle size distribution (which is somewhat broader than that in the Komuro patent, above, being as high as 10 .mu.m), the colloidal fraction must be in the range of 50% to 65% cumulative volume. In this patent, however, certain water soluble gums and/or hydrophilic materials are employed to offset the wider range of size distribution.